Add node color to popup panel
You need to get a reference to that material, then its node tree, then the principled bsdf node, and finally the color input field.
First make sure you have python tooltips enabled in your preferences :

And then you can get the path to most fields just by hovering over it or with right click &gt; Copy data path.
See this question for more information.
In your case you could access this particular field from this particular material with :
import bpy


def draw(self, context):
    layout = self.layout

    # The name is case-sensitive :
    mat = bpy.data.materials[&quot;Material.005&quot;]
    # Assuming you didn't rename the bsdf shader :
    principled = mat.node_tree.nodes[&quot;Principled BSDF&quot;]  
    # Add the property in the layout. This will automatically create a color field. 
    # You can access inputs / outputs directly with their index or with their name as a string

    layout.prop(principled.inputs[0], &quot;default_value&quot;, text=&quot;Color&quot;)  # Text argument is optional

Result :